(1) Field of the Invention
The present invention relates to an optical waveguide device, and more particularly, to an optical waveguide device including a diffused waveguide formed by thermally diffusing a high-refractive material in a dielectric substrate and a light-receiving element which is disposed above the diffused waveguide and which receives a part of an optical wave propagating in the diffused waveguide.
(2) Description of the Related Art
In an optical waveguide device such as an optical modulator including an optical waveguide, a part of optical waves propagating in the optical waveguide is directly monitored (referred to as “in-phase monitoring method”) or a radiation-mode beam radiated from the optical waveguide such as an optical Y-junction coupler of the optical waveguide is monitored. For example, to keep an output beam of an optical modulator in a constant output status, the output beam of the optical modulator is monitored and the magnitude of a modulation voltage or a DC bias applied to the optical modulator is controlled on the basis of the variation of the output beam.
In an optical modulator including a Mach-Zehnder type optical waveguide, a method of monitoring a radiation-mode beam radiated from an optical Y-junction coupler used for monitoring a bias point of the modulator has an advantage that the loss of a signal beam is suppressed, but has the following disadvantages. (1) A signal beam and a monitored beam are reverse in phase and a phase difference is deviated from π, (2) A structure for efficiently taking out the radiation-mode beam is complex and it is difficult to align a light-receiving element on a substrate, thereby making a decrease in size or a decrease in cost difficult, and (3) in a multi-stage optical modulator including plural optical modulators, it is difficult to accurately monitor the radiation-mode beam by the use of portions other than the final-stage optical Y-junction coupler.
On the contrary, in an in-phase monitoring method of directly monitoring a part of optical waves propagating in an optical waveguide, there is no phase difference from a signal beam and it is possible to monitor the signal beams of the optical modulators by the use of portions other than the final-stage optical Y-junction coupler in the multi-stage optical modulator.
Examples of the in-phase monitoring method include a method of forming a slit in a part of an optical waveguide and receiving a reflected beam with a mirror as described in Patent Literature 1, a method of generating a radiated beam in an S-shaped optical waveguide and receiving the radiated beam as described in Patent Literature 2, and a method of forming a hole having a conical shape or the like in the optical waveguide, filling the hole with a high-refractive material, and guiding and receiving optical waves to the upside of the optical waveguide as described in Patent Literature 3.
In these methods, since all the beams extracted from the guided beams cannot be received in principle, there is a problem in that the optical power which can be received by a light-receiving element and the loss of the guided beams is great, that is, the excessive loss is great.
On the other hand, an evanescent coupling light-receiving element has been suggested. A light-receiving element (high-refractive substrate of the light-receiving element with a refractive index np) having a refractive index higher than that of the optical waveguide (with an effective refractive index nf) is disposed close to the optical waveguide and an evanescent wave is input to the light-receiving element at an angle of sin−1(nf/np) about the waveguide. It is possible to detect the evanescent wave by disposing a light-receiving portion of the light-receiving element in an optical path of an incident beam.
The evanescent coupling light-receiving element has an advantage that the excessive loss can be made to converge on 0% theoretically by a design of the light-receiving element. The light-receiving sensitivity ((optical power received by light-receiving element)/(optical power propagating in waveguide)) is determined depending on the length of a part contacting the optical waveguide and a gap between the optical waveguide and the light-receiving element. Accordingly, when the shape of the light-receiving element is determined, it is possible to adjust the received optical power by adjusting the gap between the optical waveguide and the light-receiving portion (light-receiving element).
As described in Patent Literature 4, a semiconductor waveguide device has been suggested as an example of the evanescent coupling light-receiving element. In the semiconductor waveguide device, since the optical waveguide or the light-receiving element is formed by the crystalline growth, it is possible to control the thicknesses of layers with high precision and to reproducibly form the structure, thereby guaranteeing stable received optical power.
On the contrary, to implement an evanescent coupling light-receiving element in the diffused waveguide formed on a dielectric substrate, it is considered that a light-receiving element is bonded to the surface of the optical waveguide with an adhesive or by direct bonding. However, as described above, the light-receiving sensitivity is determined depending on the gap between the optical waveguide and the light-receiving element formed of a high-refractive material for absorbing light. Accordingly, a high-precision gap adjustment is inevitable for stably maintaining the light-receiving sensitivity. In the diffused waveguide, the surface of the optical waveguide swells and is thus not flat. Accordingly, it is difficult to control the gap between the diffused waveguide and the light-receiving element. A certain thickness and a certain area of the adhesive are necessary for bonding the light-receiving element to the dielectric substrate with high reliability and satisfactory strength.
[Citation List]
[Patent Literatures]                [Patent Literature 1] JP-A-2006-47894        [Patent Literature 2] JP-A-5-224044        [Patent Literature 3] JP-A-11-194237        [Patent Literature 4] JP-A-2005-129628        